The present invention relates generally to data communications. The invention more particularly relates to timing recovery techniques used in data receivers which have automatic and/or adaptive equalizers.
Accurate reception of high-speed data signals transmitted over a bandlimited channel with unknown transmission characteristics requires the use of an automatic equalizer. The equalizer, which is resident in the receiver portion of a data set, or modem, is generally in the form of a transversal filter. Samples of the received data signal, referred to herein as line samples, are formed at a predetermined sampling rate. In a so-called T/2 equalizer, for example, the line samples are formed at twice the transmitted symbol rate. The line samples are applied to the transversal filter, in which each is multiplied by respective one of a queue of coefficients. The resulting products are added together and, if necessary, demodulated to generate a baseband signal, referred to herein as an equalizer output. The value of each equalizer output is used as the basis for forming a decision as to the value of a respective transmitted data symbol.
In addition, an error signal is formed equal to the difference between each equalizer output and a reference signal which represents its respective data symbol. In the so-called adaptive type of automatic equalizer, in particular, the reference signal is derived from above-mentioned decision. The error signal is used to update, or adapt, the transversal filter coefficient values in such a way as to minimize a measure of the channel-induced distortion--assumed to be primarily intersymbol interference--in the equalizer outputs.
An important equalizer operating parameter, in addition to the rate at which the line samples are formed, is their time occurrence with respect to the received signal. This parameter is referred to as the timing epoch. The coefficient values subsisting in the equalizer at any given time will yield accurate equalization only if the received signal is sampled at or near a particular set of time points on the received signal, i.e., only if it is sampled with the appropriate timing epoch. The frequencies of the transmitter and receiver clocks invariably differ from one another, if only by a very small amount. Over time, this frequency difference, if not compensated for, would cause the received signal to be sampled further and further away from the appropriate time points, i.e., with an increasingly erroneous timing epoch. As long as the sampling frequency is high enough, the equalizer does have the ability to compensate for this clock frequency difference (as long as it is not too large) via the coefficient update process. This is not an effective long-term solution, however, because the distribution of coefficient values will eventually become skewed to one end of the coefficient queue and equalizer performance will degrade sharply.
To deal with this problem, the receiver is conventionally provided with a so-called timing recovery circuit. The timing recovery circuit determines whether the line samples are being formed earlier (later) than they should be and, in response, adjusts the phase of the line sample forming circuitry such that the line samples are formed a little later (sooner) than they otherwise would. This phase adjustment process is referred to as retarding (advancing) the receiver timing or, alternatively, as retarding (advancing) the sampling phase.
A commonly used timing recovery technique is so-called envelope-derived timing recovery, disclosed, for example, in the Bell System Technical Journal, Vol. 54, p. 569 et seq, March, 1975. This technique extracts a symbol-rate tone from the received signal and uses the phase of that tone to control receiver timing. Envelope-derived timing recovery performs satisfactorily for many applications. In some situations, however--such as a narrow rolloff system--the recovered tone may be so weak that accurate timing recovery is not possible when random data is being received.
An alternative timing recovery technique, referred to herein as "coefficient tracking," controls receiver timing as a function of coefficient distribution within the queue. In accordance with a typical coefficient tracking approach disclosed, for example, in U.S. Pat. No. 4,004,226, issued Jan. 18, 1977 to S. U. H. Qureshi et al., timing is adjusted such that the largest coefficient magnitude tends to remain at a designated "center" position in the coefficient queue. This is achieved by periodically, e.g., in each symbol interval, identifying the coefficient which has the largest magnitude. If it is found to be the center coefficient, no timing adjustment is made. Otherwise, the receiver timing is advanced or retarded, as appropriate, such that subsequent coefficient adaptation over a number of symbol intervals causes the magnitude of the center coefficient to ultimately become the largest.